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Moon and Saturn

StarDate:

May 4, 2015

The almost-full Moon arcs low across the south tonight. It’s accompanied by the planet Saturn, which looks like a bright golden star. Saturn is close to the lower left of the Moon at nightfall, and even closer to the Moon at first light tomorrow.

Because of our changing perspective on the Moon, it appears to rotate as it crosses the sky. At nightfall, the north pole is at the left or upper left of the lunar disk. When the Moon stands highest in the sky, around 1 or 2 in the morning, the north pole is at the top of the disk. And shortly before it drops from view, the north pole is at the upper right.

It’s one of those views of the sky that’s a bit deceptive. Although it looks like the Moon is flipping over, it’s really not. The change in angle is the result of our changing perspective — we’re viewing the Moon from different angles at different times.

In fact, the night sky is full of these optical illusions.

Saturn is another example. Right now, it shines brighter in our night sky than all but a handful of stars and planets. But that’s only because Saturn is quite close by. The true stars visible to the unaided eye are millions of times farther away. At such great range, the stars have to be hundreds of millions of times brighter than Saturn is for us to see them at all.

One of the brightest stars in the whole galaxy stands below Saturn as they climb into good view: Antares, the heart of the scorpion. More about that tomorrow.

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